This is a prospective clinical study designed to evaluate the rate of progression of atherosclerotic lower extremity (LED) and cerebrovascular (CVD) disease in a population of patients with LED and/or CVD. The study will examine the relationships between progression of atherosclerosis, level of plasma homocysteine (HC), and treatment to lower plasma HC (folic acid) in these patients. The study will occupy eight years; one year for enrollment and randomization of 400 patients with CVD and/or LED.